1. Field of Invention
The present invention relates to an electrooptical device and an electronic device in which a MIS transistor is formed in a semiconductor layer on an insulating layer in such a manner as to avoid floating substrate effects.
2. Description of Related Art
The SOI (silicon on insulator) technology is used to form a semiconductor device such as a transistor using a semiconductor layer such as a single-crystal silicon layer formed on an insulating material. The semiconductor devices according to the SOI technology are advantageous in that they can operate at a high speed with low power consumption and in that a high integration density can be achieved. Thus, the semiconductor devices according to the SOI technology may be advantageously applied to electrooptical devices such as a liquid crystal device.
In the case of a MIS transistor in the form of a bulk semiconductor device, the channel region of the MIS transistor is generally fixed at a particular potential via a substrate on which the MIS transistor is formed. Therefore, no degradation occurs in electrical characteristics such as a reduction in the breakdown voltage due to extrinsic bipolar transistor effects caused by a change in the potential of the channel region.
However, in SOI-type MIS transistors, because the bottom of the channel is completely isolated by the underlying insulating film, the channel region is not fixed at a particular potential and is held in an electrically floating state. If excess carriers are generated by impact ionization which occurs when carriers accelerated by the electric field near the drain region collide with the crystal lattice, the excess carriers accumulate in the bottom part of the channel. The excess carriers accumulated in the bottom part of the channel result in an increase in the potential of the channel. As a result, the NPN structure (in the case of the n-channel transistor) including the source, the channel, and the drain acts as an extrinsic bipolar device, which causes an abnormal current to flow. Thus, degradation in the electrical characteristics such as a reduction in the breakdown voltage occurs. These phenomena caused by the electrically floating channel are called floating substrate effects.
One known technique to avoid the above problems due to the floating substrate effects is to form a body contact region electrically connected to the channel region via a particular path, thereby removing the excess carriers from the channel region, so as to suppress the floating substrate effect.
However, if a body contact region is formed in a MIS transistor used in a pixel area of an electrooptical device such as a liquid crystal device, the body contact region makes it difficult to produce pixels with a high density. In particular, in the case of a transmissive electrooptical device, the body contact region results in a reduction in the aperture ratio. Also, in the case where a body contact region is formed in a peripheral driving circuit in an area other than the pixel region, the body contact region results in a reduction in the integration density. In electrooptical devices used in electronic devices such as a projection-type display device, when high-intensity light is incident on pixels, carriers are generated by optical excitation. This causes a loss in the charge stored in pixel storage capacitors. As a result, non-uniformity such as flicker occurs in a displayed image.
In view of the above, it is an object of the present invention to provide an electrooptical device having a transistor in which floating substrate effects peculiar to SOI are suppressed, particularly for use in an electronic device in which the problem of leak current induced by optical excitation is notable, e.g., a projection-type display device, as well as an electronic device incorporating such an electrooptical device.
In order to achieve the above objects, the present invention provides a semiconductor device formed in a semiconductor layer on an insulating layer according to various aspects as described below.
According to a first aspect of the present invention, there is provided an electrooptical device comprising a substrate including a base substrate, a first insulating layer formed on the base substrate, and a semiconductor layer formed on the first insulating layer, the electrooptical device further comprising the following elements formed on the substrate: a plurality of scanning lines, a plurality of data lines extending so as to cross the plurality of scanning lines; pixel transistors each connected to one of the plurality of scanning lines and also to one of the plurality of data lines; pixel electrodes connected to the respective pixel transistors; and a peripheral circuit including a driving transistor for driving the pixel transistors, the electrooptical device including a pixel transistor or a driving transistor in which at least either a region including the boundary between a channel region and a source region or a region including the boundary between the channel region and a drain region has a defect density higher than that of the channel region.
In the pixel transistors or the driving transistor according to the first aspect of the present invention, the defects in the region with the higher defect density than the channel region acts as a carrier recombination center. As a result, accumulation of excess carriers is prevented and the floating substrate effects are suppressed.
According to a second aspect of the present invention, there is provided an electrooptical device comprising a substrate including a base substrate, a first insulating layer formed on the base substrate, and a semiconductor layer formed on the first insulating layer, the electrooptical device further comprising the following elements formed on the substrate: a plurality of scanning lines, a plurality of data lines extending so as to cross the plurality of scanning lines; pixel transistors each connected to one of the plurality of scanning lines and also to one of the plurality of data lines; and pixel electrodes connected to the respective pixel transistors, the pixel transistors being formed such that at least either a region including the boundary between a channel region and a source region or a region including the boundary between the channel region and a drain region has a defect density higher than that of the channel region.
In the pixel transistors according to the second aspect of the present invention, the defects in the region with the higher defect density than the channel region acts as a carrier recombination center, which prevent accumulation of excess carriers, thereby allowing the floating substrate effects to be suppressed without forming a body contact. Thus, it is possible to achieve an electrooptical device with a high aperture ratio.
According to a third aspect of the present invention, there is provided an electrooptical device comprising a substrate including a base substrate, a first insulating layer formed on the base substrate, and a semiconductor layer formed on the first insulating layer, the electrooptical device further comprising the following elements formed on the substrate: a plurality of scanning lines, a plurality of data lines extending so as to cross the plurality of scanning lines; pixel transistors each connected to one of the plurality of scanning lines and also to one of the plurality of data lines; pixel electrodes connected to the respective pixel transistors; and a peripheral circuit including a driving transistor for driving the pixel transistors, the pixel transistors or the driving transistor being formed such that at least either a region including the boundary between a channel region and a source region or a region including the boundary between the channel region and a drain region has a defect density higher than that of the channel region.
In the pixel transistors or the driving transistors according to the third aspect of the present invention, the defects in the region with the higher defect density than the channel region act as a carrier recombination center, which prevent accumulation of excess carriers, thereby allowing the floating substrate effects to be suppressed without forming a body contact. Thus, it is possible to achieve an electrooptical device with a great aperture ratio. Furthermore, it becomes possible to layout a peripheral circuit in a highly efficient fashion.
According to a fourth aspect of the present invention, there is provided an electrooptical device comprising a substrate including a base substrate, a first insulating layer formed on the base substrate, and a semiconductor layer formed on the first insulating layer, the electrooptical device further comprising the following elements formed on the substrate: a plurality of scanning lines, a plurality of data lines extending so as to cross the plurality of scanning lines; pixel transistors each connected to one of the plurality of scanning lines and also to one of the plurality of data lines; pixel electrodes connected to the respective pixel transistors; and a peripheral circuit including a driving transistor for driving the pixel transistors, the electrooptical device including a pixel transistor or a driving transistor in which at least a region extending toward a channel region from the boundary between the channel region and a source region or a region extending toward the channel region from the boundary between the channel and a drain region has a higher defect density than that of the channel region.
In the pixel transistors or the driving transistors according to the fourth aspects of the present invention, the defects in the region with the higher defect density than the channel region acts as a carrier recombination center. As a result, accumulation of excess carriers is prevented and the floating substrate effects are suppressed.
According to a fifth aspect of the present invention, there is provided an electrooptical device comprising a substrate including a base substrate, a first insulating layer formed on the base substrate, and a semiconductor layer formed on the first insulating layer, the electrooptical device further comprising the following elements formed on the substrate: a plurality of scanning lines, a plurality of data lines extending so as to cross the plurality of scanning lines; pixel transistors each connected to one of the plurality of scanning lines and also to one of the plurality of data lines; and pixel electrodes connected to the respective pixel transistors, the pixel transistors being formed such that at least a region extending toward a channel region from the boundary between the channel region and a source region or a region extending toward the channel region from the boundary between the channel and a drain region has a higher defect density than that of the channel region.
In the pixel transistors according to the fifth aspect of the present invention, the defects in the region with the higher defect density than the channel region acts as a carrier recombination center, which prevent accumulation of excess carriers, thereby allowing the floating substrate effects to be suppressed without forming a body contact. Thus, it is possible to achieve an electrooptical device with a high aperture ratio.
According to a sixth aspect of the present invention, there is provided an electrooptical device comprising a substrate including a base substrate, a first insulating layer formed on the base substrate, and a semiconductor layer formed on the first insulating layer, the electrooptical device further comprising the following elements formed on the substrate: a plurality of scanning lines, a plurality of data lines extending so as to cross the plurality of scanning lines; pixel transistors each connected to one of the plurality of scanning lines and also to one of the plurality of data lines; pixel electrodes connected to the respective pixel transistors; and a peripheral circuit including a driving transistor for driving the pixel transistors, the pixel transistors or the driving transistor being formed such that at least a region extending toward a channel region from the boundary between the channel region and a source region or a region extending toward the channel region from the boundary between the channel and a drain region has a higher defect density than that of the channel region.
In the pixel transistors according to the second aspect of the present invention, the defects in the region with the higher defect density than the channel region acts as a carrier recombination center, which prevent accumulation of excess carriers, thereby allowing the floating substrate effects to be suppressed without forming a body contact. Thus, it is possible to achieve an electrooptical device with a high aperture ratio. Furthermore, it becomes possible to layout a peripheral circuit in a highly efficient fashion.
In the electrooptical device according to any of the foregoing aspects of the present invention, the pixel transistors each connected to one of the plurality of scanning lines and also to one of the plurality of data lines are preferably p-channel transistors. In p-channel transistors, holes behaving as minority carriers have a smaller impact ionization coefficient than electrons. Therefore, p-channel transistors provide less floating substrate effects than n-channel transistors, and thus p-channel transistors can be driven with a higher voltage without using a body contact than n-channel transistors. Thus, by employing p-channel transistors as the pixel transistors, it becomes possible to achieve an electrooptical device having a greater aperture ratio. Furthermore, in the transistors each connected to one of the scanning lines and also connected to one of the data lines, the defects, which are produced in the particular regions having a higher defect density than the channel region acts as carrier a recombination center, thereby preventing excess carriers from accumulating. As a result, the floating substrate effects are suppressed. Thus, the transistors according to the present aspect of the invention are suitable for driving the liquid crystal or the like which requires a high driving voltage. Furthermore, in the described aspects of the invention, it is desirable that the semiconductor layer formed on the first insulating layer have a thickness equal to or less than 100 nm, at least at locations where the pixel transistors each connected to one of the plurality of scanning lines and also to one of the plurality of data lines are formed. That is, the small thickness of the semiconductor layer at locations where the pixel transistors each connected to one of the plurality of scanning lines and to one of the plurality of data lines are formed (that is, at locations which are illuminated with light) allows the leakage current due to optical excitation to be minimized.
In any of the foregoing aspects of the present invention, the defects in the high-defect-density regions are preferably produced by means of implantation of Ar ions so that the produced defects act as recombination centers.
In any of the foregoing aspects of the present invention, it is desirable that the base substrate is made of single-crystal silicon. This allows the electrooptical device to be applied to a reflective liquid crystal device or the like. Another advantage is that the electrooptical device can be produced simply by using production apparatus for bulk silicon devices.
In any of the foregoing aspects of the present invention, the base substrate may be made of quartz and the semiconductor layer on the first insulating layer may be made of single-crystal silicon. In this case, the base substrate is transparent, and thus the electrooptical device may be applied to a transmissive liquid crystal or the like. Another advantage is that a high-quality insulating film or the like can be formed and thus a high-reliability device can be realized using a high-temperature process which is not possible when the base substrate is made of glass. Furthermore, because the semiconductor layer is formed of single-crystal silicon, it becomes possible to realize a high-quality, high-precision electrooptical device capable of operating at a high driving frequency.
In any of the foregoing aspects of the present invention, the base substrate may be made of quartz and the semiconductor layer on the first insulating layer may be made of polycrystalline silicon. Also, in this case, the base substrate becomes transparent, and thus the electrooptical device may be applied to a transmissive liquid crystal or the like. Furthermore, a high-quality insulating film or the like can be formed and thus a high-reliability device can be realized using a high-temperature process which is not possible when the base substrate is made of glass. Furthermore, the semiconductor layer made of polycrystalline silicon has the advantage that it can be easily formed on a substrate, and thus a high-precision electrooptical device can be easily produced.
In any of the foregoing aspects of the present invention, the base substrate may be made of glass. In this case, because the base substrate is formed so as to be transparent using low-cost glass, it is possible to realize a transmissive electrooptical device such as a liquid crystal device at low cost.
According to still another aspect of the present invention, there is provided an electronic device comprising: a light source; an electrooptical device according to one of the aspects described above, for modulating light, which falls upon the electrooptical device after being emitted from the light source, in accordance with image information; and projection means for projecting the light modulated by the electrooptical device.